MCOL-3713: Join abort problem

Noticed a race on djsJoiners in abort() and segregateJoiners().
Re-added the mutex lock, but after risk of double lock grab.

dbcon/joblist/tuplehashjoin.cpp

@@ -1938,6 +1938,7 @@ void TupleHashJoinStep::segregateJoiners()
     }
     #endif
 
+    boost::mutex::scoped_lock sl(djsLock);
     /* For now if there is no largeBPS all joins need to either be DJS or not, not mixed */
     if (!largeBPS)
     {